Gas turbines typically operate on natural gas fuel, with fuel oil (typically no. 2 distillate) often used as a contingency for periods of gas unavailability. When a gas turbine is operating on natural gas fuel, the fuel oil typically remains in liquid fuel lines (for example, piping/tubing) leading to the combustor nozzles of the gas turbine. The stagnate fuel oil in a liquid fuel lines is often exposed to the turbine compartment air temperatures of up to 200° F., and turbine surfaces of up to 800° F.
Typically, a gas turbine has a number of combustors positioned around the turbine, each combustor having a gas fuel nozzle and liquid fuel nozzle. When the turbine is running on one type of fuel, the nozzle for the other type of fuel must be purged of the other type of fuel. Thus, for example, when a turbine is switched from running on fuel oil back to running on natural gas fuel, the fuel oil and the liquid fuel nozzle must be purged. Over time, this fuel nozzle, “purge air” fills some portion of the liquid fuel piping leading up to the liquid fuel nozzle as the level of fuel oil in the piping recedes due to leakage past upstream shut-off valves, and by thermal expansion and contraction with no make-up supply of liquid fuel. This air/oil interface on the coated surfaces of the piping system and valves (for example, check valves, ball valves, spool valves, purge valves, etc.) in the presence of the radiated, conducted and convected heat, leads to coke formation in the liquid fuel piping, resulting in flow restriction and inoperable valves. Eliminating any one of the three ingredients required for a coke formation (i.e., fuel oil, heat and air) will prevent coking. Since it not practical to eliminate fuel or air in a turbine, it would be beneficial to eliminate the heating of the liquid fuel lines, thereby resulting in the prevention of coking in the liquid fuel line piping and valves.
Prior attempts have been made to direct turbine compartment cooling air flow to the areas subject to coking, but sufficient temperature cooling could not be obtained. Typically, a combustor in the turbine operates at a temperature well over 2000° F. The heat from the combustor radiates towards compartments, such as the fuel, oil, piping and valves, sitting in the turbine enclosure. Even with attempts to ventilate the enclosure that included directing cooling air toward components subject to coking, air temperatures of 300° F. around such components was still typical.
There remains a need, therefore, for an efficient manner of cooling fuel components, and particularly gas turbine three-way purge valves, subject to high heat in the area of a gas turbine combustor.